The human body is a biological movement machine designed to maintain a centralized center of gravity inside its base of support (hereinafter “BOS”). Skeletal bones of the body form the framework, while skeletal muscles move the framework. Tendons found at the ends of skeletal muscles attach the skeletal muscles to the skeletal bones and help maintain the postural alignment of the body. Ligaments attach bone to bone, and have a limited amount of flexibility in order to maintain the attachment of the skeletal bones in the framework.
The articulating bones of the body that form joints stay aligned and positioned properly using skeletal muscles, ligaments, tendons, and fascia. Locomotion that keeps the joints aligned as designed and inside the body's natural BOS also keeps the skeletal muscles and fascia strong and flexible, and helps the body produce synovial fluid. Synovial fluid lubricates, shock absorbs and reduces friction on joints. It also brings nutrients to joints and removes carbon dioxide and metabolic waste.
When the joints of the human body are in postural alignment during locomotion, the body stays within its BOS and maintains a low center of gravity (hereinafter “COG”). The upward support force from the BOS aligns with the downward force of gravity. The stability of the body during locomotion depends on the gravitational balance and stability of the arms and legs. Injury or movement of a joint outside of the body's natural BOS creates overloading or under loading to all other joints due to the redistribution of forces. Under loading or over loading of a joint or movement that causes hyperextension of a joint or its supporting tissue can result in a loss of physical stability and postural alignment. Overtime, repetitive movement that doesn't maintain the body's COG over its base can result in physical and functional disability. The Specific Adaptation to Imposed Demands (“SAID”) principle states that the body will gradually adapt to stresses and overloads that it is subjected to. Wolff's Law states that bone function changes cause bone structure modification. Davis's Law states that soft tissue's tendency is to shorten and contract unless subject to frequent stretching; in other words, and to quote Dr. Davis, “[u]se it or lose it.” Hook's Law states that tissue strain is directly proportional to applied compressive or stretching stress so long as tissue elasticity is not exceeded.
The general principles of balance and stability include the following:
1. Gravity intersects the BOS of the subject;
2. Anything that decreases the BOS decreases stability of the subject;
3. The lower the COG above the BOS, the more stability of the subject;
4. Objects that have more mass over or near the COG tend to be more stable;
5. The farther the COG intersection line is from the edge of the BOS, the more stable the subject;
6. Stability is directly proportional to the area of the BOS on which a body rests;
7. Stability in a given direction is directly proportional to the horizontal distance of the COG from the edge of the BOS;
8. When two objects have a different shape, but an equal mass, the one with the wider base will be more stable;
9. The further the COG is from the direction of movement, the more likely it is to maintain stability;
10. When a body has balance and physical stability, it has equilibrium, and the COG is inside the BOS;
11. When the BOS is widened in the direction of the line of force (hereinafter “LOF”), the body has greater stability. When the BOS is widened laterally on one side of the body, the COG move closer to the edge and the body has less stability; and
12. Postural stability occurs when the COG and the LOF are over the center of pressure (COP).
Now, relating these principles to walking, during the normal gait cycle the arms as well as the rest of the body stay within the body's natural BOS to maintain the body's balance. The heel makes contact with the ground before the rest of the foot. The body's COG is over and inside its BOS. The shoulder and hip joints maintain vertical orientation and alignment with the pectoral and pelvic girdles. After the heel contacts the ground, the rest of the foot rolls onto an over the ground. The body's weight then passes over its COG as the heel lifts off the ground and the body moves forward. When the gait cycle has reciprocal movement, the shoulder, hip, knee and ankle joints work together to load the weight of the body over and on the foot within the body's BOS. The head stays positioned over the body and the line of sight is in the direction the person is moving.
During locomotion, the human foot has two functions. First, during the stance phase of the gait cycle the foot acts as a mobile adaptor and shock absorber to maintain the body's balance and physical stability on uneven surfaces or terrain. Second, during the swing phase of gait the foot lifts off the ground completely and acts as a lever to propel the body forward. A lever is a rigid bar resting on a pivot, used to help move a heavy or fixed load with one end when pressure is applied to the other.
Canes extend the BOS on one side of the body and act as a substitute for the hip flexors on that side by transmitting part of the body's weight to the ground. In order to have a mechanical advantage when using a traditional cane, the distance between the axis of the hip joint and the contralateral hand must be extended away from the body. The mean position of the COP shifts laterally toward the cane side to maintain the body's balance. However, when the COP moves laterally on one side only, the COG of the body moves higher and closer to the edge of the BOS. Due to the size, shape, and orientation of the traditional cane foot and its orientation with the cane shaft, the person's arm, wrist and head shift forward and downward during locomotion. When the head is positioned downward during locomotion, the line of sight is towards the ground. This causes a reduction to both sensory and proprioceptor input which results in a loss of afferent messages traveling from the body to the brain, as well as motor responses traveling from the brain back to the body. In order to maintain vertical orientation of the body during locomotion when the head and line of sight are positioned towards the ground, the length of the step as well as the stride must be shortened. As a result of the shortened strides, the heel of the foot nearest the cane stops striking the ground from heel to toe and the foot loses its ability to quickly transform from a mobile adaptor to a ridged lever during the gait cycle. Instead, the downward position of the head and line of sight along with the shortened step and stride force the heels to rotate medially towards each other, while the toes rotate laterally away from the direction the person is moving to maintain the body's vertical orientation.
Use of traditional canes can also cause continuous repetitive movement that hyperextends the wrist joint outside the body's natural BOS. The head of the humerus on the side of the body using the cane loses vertical alignment and postural equilibrium with the shoulder girdle during locomotion, as does the scapula. Overtime, movement that moves the COP laterally on one side of the body only, and hyperextends the wrist joint forward away from the body's natural BOS may result in physical and functional disability and pain. When the wrist joint hyperextends, the distance between the axis of the hip joint and the contralateral hand becomes greater. A body is in equilibrium when the downward directed linear force is equal to the upward force and the vector sum of all forces equals zero. When the COP on one side of the body is located laterally and at the edge of the BOS and not centrally orientated during locomotion, the body loses postural equilibrium and the COG becomes higher and less stable because low stability of a person or object is associated with a high COG and a gravity projection at the edge or outside of the BOS.
To put this in perspective, there are twenty-six bones, thirty-three joints, and over a hundred muscles, ligaments, and tendons in each foot. Thirty of the thirty-three joints found in each foot are synovial joints. Synovial joints have no blood supply of their own, and therefore rely on movement in and around the joint to maintain adequate levels of synovial fluid inside the joint. The size, shape, alignment, and positioning of the traditional cane foot with the cane shaft and handle keeps the body's weight from rolling over the foot nearest the cane from heel to toe. When the body weight does not roll over and onto the foot from heel to toe, the synovial joints of the foot stop producing adequate amounts of synovial fluid, and the muscles and fascia around the synovial joints of the feet contract and the feet can become painful.
There remains a need for an ambulatory aid or cane having a cane foot that more closely functions to mirror the normal gait cycle of the musculoskeletal system.